Rashi on Deuteronomy 33:21/רש"י על דברים ל״ג:כ״א

1וירא ראשית לו AND HE SAW THE FIRST PART FOR HIMSELF — i.e. he saw good to take for himself territory in the land of Sihon and Og, the subjugation of which was the beginning (ראשית) of the conquest of the Land,

2כי שם חלקת BECAUSE THERE WAS THE PORTION — i.e., because he knew that there in his territory was the portion of the field where was the grave of the מחקק, THE LAWGIVER, viz., Moses,

3ספון HIDDEN — i.e. that field was hidden and concealed from every creature, as it is said (Deuteronomy 34:6) “And no man knows his burial place” (Sifrei Devarim 355:6).

4ויתא AND HE CAME — i.e. Gad came.

5ראשי עם AT THE HEAD OF THE PEOPLE — They (the men of Gad) marched before the armed men at the conquest of the Land, because they were mighty, and thus does it state that Joshua said to them, (Joshua 1:14) “Ye shall pass armed before your brethren, etc.”.

6צדקת ה' עשה HE DID THE RIGHTEOUSNESS OF THE LORD — because they proved true to their word and kept their promise to cross the Jordan and to remain there until they (the Israelites) had conquered it and divided it amongst the tribes. — Another explanation: ויתא AND HE — the Lawgiver, Moses — CAME ראשי עם AT THE HEAD OF THE PEOPLE, צדקת ה' עשה HE DID THE RIGHTEOUSNESS OF THE LORD — This, too, is spoken of Moses (cf. Sifrei Devarim 355:9).